During the manufacturing of semiconductor ICs, a chip is completed typically after a series of photolithographic exposure processes. In order to ensure correct relative positioning between patterns for different IC layers, the exposure for each layer other than the first layer is preceded by a precision positioning process for aligning the pattern for the current layer with the previously exposed one. Relative positioning errors between the patterns are known as overlay errors. Generally, permissible overlay errors are required to be within one third to one fifth of the resolution of a photolithography tool. For example, a photolithography tool with a resolution of 100 nm is required to produce overlay errors smaller than 35 nm. Overlay error performance is an important measure for the quality of a projection photolithography tool and is greatly determined by its mask-to-wafer alignment accuracy. Smaller characteristic dimensions (CDs) tend to impose higher requirements on overlay error performance and hence on alignment accuracy. For example, a CD of 90 nm requires alignment accuracy of 10 nm or below.
FIG. 1 shows a dual-wafer-stage system employed by a step-and-scan photolithography tool for a higher throughout. The system includes: a main frame 101; a projection objective 102 and an alignment sensor 103, both attached to the main frame 101; a wafer stage 104 under exposure corresponding to the projection objective 102; and a wafer stage 105 under measurement corresponding to the alignment sensor 103. A wafer 107 under measurement is placed on the wafer stage 105, and a wafer 106 under exposure is positioned on the wafer stage 104. The wafer stage 105 is configured for measurement of the wafer, including measurements for alignment, leveling and focusing, etc., and the wafer stage 104 is adapted primarily for pattern exposure. They work in parallel and interchangeably, which can lead to a great improvement in throughout.
The employment of such a dual-stage arrangement in the dual-wafer-stage system, however, may augment the vibration of the main frame 101 and hence of the alignment sensor 103 transferred from the projection objective 102. Moreover, the increased throughput requires greater accelerations and thus greater impacts of the wafer stages, which can make a further contribution to the augmentation of the vibration of the alignment sensor 103. As a result of the augmented vibration of the alignment sensor 103, greater alignment errors may occur in the alignment mark measurement.
Furthermore, the requirements on the repeatability accuracy of alignment increase with those on the overlay accuracy of the photolithography tool. Since the vibration of the alignment sensor 103 can directly introduces alignment errors, tolerance of the alignment sensor 103 on such alignment errors will be increasingly low A repeatability accuracy of alignment of up to 2 nm requires vibration amplitude of the alignment sensor 103 of 10 nm (which will lead to an alignment error of about 0.3 nm) or less. As it is difficult to control the vibration amplitude of the alignment sensor in the dual-wafer-stage system within 10 nm, there is a problem of insufficient repeatability accuracy of alignment.